Scanning electron microscope and sample observation method

ABSTRACT

A scanning electron microscope of the present invention performs scanning by changing a scanning line density in accordance with a sample when an image of a scanned region is formed by scanning a two-dimensional region on the sample with an electron beam or is provided with a GUI having sample information input means which inputs information relating to the sample and display means which displays a recommended scanning condition according to the input and performs scanning with a scanning line density according to the sample by selecting the recommended scanning condition. As a result, in observation using a scanning electron microscope, a suitable scanning device which can improve contrast of a profile of a two-dimensional pattern and suppress shading by suppressing the influence of charging caused by primary charged particle radiation and by improving a detection rate of secondary electrons and a scanning method are provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 13/387,183, filed Mar. 1, 2012, which is a National Stage Entry ofPCT/JP2010/004843, filed Jul. 30, 2010, which claims the priority ofJapanese Application No. 2009-241966, filed Oct. 21, 2009 and JapaneseApplication No. 2009-184001, filed Aug. 7, 2009, which are incorporatedby reference herein in their entirety.

TECHNICAL FIELD

The present invention relates to, in a scanning electron microscopewhich observes a sample by using charged particle radiation, a method inwhich electrons emitted from the sample by the charged particleradiation are detected/imaged and the shape and composition of thesample are observed on the basis of that and a scanning electronmicroscope used for that purpose.

BACKGROUND ART

Due to integration of a semiconductor device and narrowing of a processmargin involved in that, the need for two-dimensional dimensionmeasurement such as a contact hole and a wiring pattern in addition tothe prior-art line and space measurements (one-dimensional lengthmeasurement) is increasing for a testing/measuring device on the basisof a scanning electron microscope which observes a superfine pattern.

However, in the prior-art technologies, when a fine pattern formed of aninsulating material such as resist or SiO2 is subjected to SEMobservation, the profile line intensity of the pattern depends on theelectron scanning direction (the intensity of the pattern edge parallelwith the electron scanning direction is lowered). This is caused by thefact that the sample is charged by electron irradiation, and thus, shapedistortion or uneven brightness occurs in an image. FIG. 1 shows anexample of an electron scanning method in a view field when an image isobtained. An image of the resist pattern (FIG. 2A obtained by thisscanning method is shown in FIG. 2B. The profile line intensity of theedge of the resist and the line profile depend on the electron scanningdirection, and the profile line (parallel edge) in parallel with theelectron scanning direction might be lost in some cases. If a patternprofile line is extracted by using this image, a defect (pattern outlineloss) or an erroneous extraction (occurrence of ghost, that is, a lineis detected where there should not have been a pattern) might occur inthe profile data as illustrated in FIG. 2C. As a result, measurement ofthe original processed dimension or shape of the pattern on the sampleon the basis of profile line information becomes difficult.

The causes for that include the fact that the charged state on thesample surface is different depending on the relative direction ofelectron scanning and the pattern edge. As illustrated in FIG. 3, ifelectrons are made to scan in parallel with the pattern edge (FIG. 3A),positive charging produced by the previous scanning line is strongerthan the perpendicular scanning case (FIG. 3B), and probability ofreturn of the secondary electrons generated by electron beam irradiationto the sample surface becomes high. As a result, the profile lineintensity on the image is reduced, the profile of the pattern cannot beextracted and the dimension or shape measurement of the pattern isobstructed.

In order to suppress the above obstruction, the influence of theelectron-beam irradiation charging needs to be suppressed.Conventionally, in order to suppress the charging caused by electronirradiation, the following methods have been disclosed. For example,Patent Literature 1 describes that an inactive gas is introduced intothe vicinity of the sample and ionized by primary electron irradiation,and the charging generated on the sample surface during image taking isneutralized. Patent Literature 2 describes that irradiation charging isneutralized by a flood gun or primary electron beam (irradiation energydifferent from that in image taking) examination between frames in theimage taking. Patent Literatures 3, 4, and 5 describe that the influenceof irradiation charging is suppressed by controlling the scanningdirection of the electron beam so that the primary electron beam scansthe pattern edge perpendicularly or diagonally. Moreover, PatentLiterature 6 describes a method of optimizing the scanning interval ofthe primary electron beam in the view field for each observation sample.

CITATION LIST Patent Literature

Patent Literature 1: JP Patent No. 4057653

Patent Literature 2: U.S. Pat. No. 7,488,938

Patent Literature 3: U.S. Pat. No. 6,879,719

Patent Literature 4: U.S. Pat. No. 7,439,503

Patent Literature 5: U.S. Pat. No. 6,710,342

Patent Literature 6: JP Patent Publication (Kokai) No. 2008-123716A

SUMMARY OF INVENTION Technical Problem

In general, a charged amount caused by electron beam irradiation anddistribution of a sample including a non-metal material is known tolargely depend on the following factors:

(1) The initial charged state before the electron beam irradiation onthe sample: charged amount, distribution

(2) Energy of primary electron, probe current, observation view field,irradiation time, and secondary electron/backscattered electron yield ofthe sample caused by electron beam irradiation. Order of the irradiationpositions in the view field of electron beam.

(3) Movement, diffusion, reunion of electron/hole on non-metal materialsurface/BARC caused by electron beam irradiation

(4) Electric field/magnetic field distribution of sample peripheralregion during observation

Patent Literature 1 describes that the charging caused by primaryelectron beam irradiation is suppressed on a real time basis by usingions and electrons generated from the inactive gas and by reuniting themwith charges on the sample surface. However, this case has a problemthat the beam diameter of the primary electron becomes large due to acollision with the inactive gas, and plane resolution of the image islowered.

Also, Patent Literature 2 resets the initial charged state of the samplebefore frame irradiation for taking an image by using a flood gun orprimary electron beam. However, in this case, accurate control of thesample potential with a flood gun or primary electron beam irradiationis difficult. Also, it has a problem that a throughput of image takingis lowered by that.

Patent Literature 3, Patent Literature 4, and Patent Literature 5improve a detection rate of a secondary electron signal from the edge byrefraining from scanning with the electron beam in parallel with thepattern edge so as to suppress a coulomb force of the secondaryelectrons and the charges on the sample surface. However, in order todetermine the scanning direction, a process of obtaining an image inadvance and extracting the pattern edge is needed, and it has a problemthat the throughput of image taking is lowered. Also, in order to obtainan image high accuracy from a pattern with a small dimension and acomplicated shape, positional control in scanning with the electron beamis difficult.

In Patent Literature 6, energy of secondary electrons is discriminatedby using an energy filter, fluctuation in a sample potential is measuredfrom a change in the obtained electron yield, and a time constant ofcharging formed during the electron beam irradiation is extracted. Thescanning interval in interlace scanning is optimized on the basis of theextracted time constant, and distortion or magnification fluctuationappearing in the image is suppressed. A certain effect can be obtainedby this method but was confirmed by experiments to be insufficient forimprovement of edge intensity in an SEM image and shading suppressionfor trends to finer LSI patterns and more complicated pattern shapes.The shading is the same as ghost.

An object of the present invention is to provide a suitable scanningdevice which can improve contrast of a profile of a two-dimensionalpattern and suppress shading by suppressing the influence of chargingcaused by primary charged particle radiation and by improving adetection rate of secondary electrons in observation using a scanningelectron microscope.

Solution to Problem

In the present invention, in a scanning electron microscope which formsan image of a scanned region by scanning a two-dimensional region on asample with an electron beam, scanning is performed by changing ascanning line density in accordance with the sample.

Moreover, the present invention includes a GUI, sample information inputmeans which inputs information relating to the sample, and display meanswhich displays a recommended scanning condition by input using thesample information input means on the GUI, and the scanning is performedwith the scanning line density in accordance with the sample byselection of the recommended scanning condition.

Moreover, the present invention includes means which measures anelectric characteristic of the sample, and scans by changing thescanning line density on the basis of the measured electriccharacteristic.

Moreover, in the present invention, the scanning is performed bycontrolling at least one of the scanning order of a plurality ofscanning lines for scanning and time intervals between the scanninglines on the basis of the electric characteristic.

Moreover, in the present invention, the electric characteristic is acharging relaxation time constant of the sample calculated on the basisof a temporal change of the intensity of secondary charged particlesemitted from the sample by radiating an electron beam to the sample.

Moreover, in the present invention, the electric characteristic is acharging relaxation time constant of the sample calculated on the basisof the temporal change of the intensity of the secondary chargedparticles emitted from the sample by radiating the electron beam to aplurality of spots on the sample.

Moreover, in the present invention, the scanning line density controlsthe scanning speed of the electron beam and/or the current of theelectron beam.

Moreover, in the present invention, the scanning line density is7.2×10⁻¹⁹ (C/nm) or less.

Moreover, in the present invention, the scanning line density is3.52×10⁻¹⁹ (C/nm) or less.

Moreover, in the present invention, the scanning line density is3.2×10⁻¹⁹ (C/nm) or less.

Moreover, in the present invention, (signal/noise) is calculated from animage, and the number of frames is calculated in accordance with thecalculated value.

Moreover, in the present invention, a focal point or an astigmatismcorrection amount is calculated and the result is fed back to a chargedparticle optical system.

Moreover, in the present invention, a sample observation method in whichan image for a scanned region is formed by scanning a two-dimensionalregion on a sample with the electron beam, the scanning is performed bychanging the scanning line density in accordance with the sample.

Moreover, in the present invention, the scanning is performed by meansof a process of inputting information relating to the sample, a processof displaying a recommended scanning condition on the basis of theinformation relating to the sample on a GUI, and a scanning line densityaccording to the sample by selecting the recommended scanning condition.

Moreover, in the present invention, an electric characteristic of thesample is measured, and the scanning is performed by changing thescanning line density on the basis of the measured electriccharacteristic.

Moreover, in the present invention, the scanning is performed bycontrolling at least one of the scanning order of a plurality ofscanning lines for scanning and time intervals for scanning between thescanning lines on the basis of the electric characteristic.

Moreover, in the present invention, the electric characteristic is acharging relaxation time constant of the sample calculated on the basisof a temporal change of the intensity of secondary charged particlesemitted from the sample by radiating an electron beam to the sample.

Moreover, in the present invention, the electric characteristic is acharging relaxation time constant of the sample calculated on the basisof the temporal change of the intensity of the secondary chargedparticles emitted from the sample by radiating the electron beam to aplurality of spots on the sample.

Moreover, in the present invention, the scanning line density controlsthe scanning speed of the electron beam and/or the current of theelectron beam.

Moreover, in the present invention, the scanning line density is7.2×10⁻¹⁹ (C/nm) or less.

Moreover, in the present invention, the scanning line density is3.52×10⁻¹⁹ (C/nm) or less.

Moreover, in the present invention, the scanning line density is3.2×10⁻¹⁹ (C/nm) or less.

Moreover, in the present invention, (signal/noise) is calculated from animage, and the number of frames is calculated in accordance with thecalculated value.

Moreover, in the present invention, a focal point or an astigmatismcorrection amount is calculated and the result is fed back to a chargedparticle optical system.

Advantageous Effects of Invention

According to the present invention, in observation using a scanningelectron microscope, a suitable scanning electron microscope which canimprove contrast of a profile of a two-dimensional pattern and suppressshading by suppressing the influence of charging caused by primarycharged particle radiation and by improving a detection rate ofsecondary electrons and a sample observation method can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates interlace scanning FIG. 2 FIGS. 2A-2E are an exampleof profile extraction of a resist gap pattern by using a scanningmethod.

FIG. 3 FIGS. 3A and 3B are an explanatory diagram of influences on apattern edge of a scanning direction of an electron beam, chargingdistribution and on a secondary electron orbit.

FIG. 4 is an explanatory diagram of a coulomb force action by a chargeon the non-metal material surface.

FIG. 5 is an explanatory diagram of a device configuration of Embodiment1.

FIG. 6 is an explanatory diagram on extraction of a charging relaxationtime constant from a temporal change of image brightness in a chargedregion.

FIG. 7 is a configuration diagram until a primary electron beam scanningmethod is determined.

FIG. 8 is a sequence for obtaining an image in Embodiment 1.

FIG. 9 is an explanatory diagram of a device configuration of Embodiment2.

FIG. 10 is a sequence for obtaining an image in Embodiment 2.

FIG. 11 is a sequence for obtaining an image in Embodiment 3.

FIG. 12 is a sequence for obtaining an image in Embodiment 4.

FIG. 13 is a GUI for obtaining an image in Embodiment 5.

FIG. 14 is a diagram illustrating a relationship between an electronbeam incident line density and secondary signal detection efficiency inEmbodiment 6.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below in detailusing the attached drawings. The embodiments according to the presentinvention are only examples in realizing the present invention, and thepresent invention is not limited by these.

In the embodiments of the present invention, a method for determining anelectron beam irradiation method which suppresses the influence ofcharging of electron beam irradiation on the basis of a measurementresult of charging relaxation characteristics caused by electron beamirradiation (time constant) of the sample and a scanning electronmicroscope provided with that are provided.

The behaviors of secondary electrons and backscattered electrons emittedfrom the sample caused by electron beam irradiation largely depend onthe charged amount and distribution on the sample in the vicinity of anirradiated area. Particularly, since motion energy of the secondaryelectrons predominant in the secondary signal is small (several eV), theorbit is largely changed by the coulomb force with the chargesaccumulated on the sample. The coulomb force received by the secondaryelectrons at the irradiation position is considered as follows.

If uniform charges (charge line density λ0) on the x-axis (x=±1)illustrated in FIG. 4 are distributed on the surface of an insulatingfilm on an Si substrate, the potential at a position (0 y 0) iscalculated by the formula (1)

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \mspace{625mu}} & \; \\{{\Delta \; V} \propto {\frac{\lambda_{0}}{2{\pi ɛ}_{0}} \cdot \left( {{\int_{0}^{+ l}\frac{dx}{\sqrt{y^{2} + x^{2}}}} - {\int_{0}^{+ l}\frac{dx}{\sqrt{y^{2} + \left( {2\; h} \right)^{2} + x^{2}}}}} \right)}} & (1)\end{matrix}$

where λ0: line density of charges on the sample surface immediatelyafter electron beam irradiation (C/m)

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack \mspace{625mu}} & \; \\{\lambda_{0} = {\lambda_{incident} \times \left( {{{secondary}\mspace{14mu} {signal}\mspace{14mu} {yield}} - 1} \right)}} & (2) \\{\left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack \mspace{625mu}} & \; \\{\lambda_{incident} = \frac{{Probe}\mspace{14mu} {current}\mspace{14mu} I_{p}\mspace{14mu} {of}\mspace{14mu} {primary}\mspace{14mu} {electron}\mspace{14mu} {beam}}{{Scanning}\mspace{14mu} {{speed}\;}_{V}\mspace{11mu} {of}\mspace{14mu} {primary}\mspace{14mu} {electron}\mspace{14mu} {beam}}} & (3)\end{matrix}$

2l: View field size (m),

h: Film thickness of insulating film (m)

Similarly, the electric field intensity E_(y) at the position (0 y 0)can be expressed by the formula (2).

$\begin{matrix}{\left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack \mspace{625mu}} & \; \\{E_{y} \propto {{\frac{\lambda_{0}}{2{\pi ɛ}_{0}} \cdot \frac{d}{dy}}\left( {{\int_{0}^{+ l}\frac{dx}{\sqrt{y^{2} + x^{2}}}} - {\int_{0}^{+ l}\frac{dx}{\sqrt{y^{2} + \left( {2\; h} \right)^{2} + x^{2}}}}} \right)}} & (4)\end{matrix}$

According to the formulas (2), (3), and (4), reduction of the scanningline density λ incidence of the primary charged particle radiation iseffective in suppressing the coulomb force affecting the orbit of thesecondary electrons.

In the observation method of the sample according the present invention,by setting the scanning line density λ incidence when the sample isirradiated with the primary charged particles to 7.20×10⁻¹⁹ (C/nm) orless, the throughput is improved, and the horizontal lines becomevisible. It was confirmed by experiments that the scanning condition ofthe primary charged particle radiation is preferably determined so as torealize reduction to 3.52×10⁻¹⁹ (C/nm) or less, the throughput isimproved, and the horizontal lines become clearer and have the samecontrast as the vertical lines.

Moreover, an exchange time change of the charges caused by electron beamirradiation is different depending on the material and structure of thesample in general. In order to suppress mutual coulomb action ofsecondary electrons and sample charging, measurement of the temporalchange of charging for each sample and selection of an optimal scanningmethod for that are needed in principle. Thus, the observation method ofthe sample according to the present invention includes means whichdisperse energy of secondary electrons emitted from the sample byprimary charged particle radiation by using an energy filter andcalculates the charging relaxation time constant of the sample from thetemporal change of intensity of the secondary electrons having specificenergy and scanning-order determining means which determines the orderof scanning with the primary charged particle radiation on the sample onthe basis of the calculated time constant, and by including a process inwhich the sample is scanned with the determined scanning line densityand scanning order.

The scanning electron microscope according to the present invention is ascanning electron microscope which radiates the primary charged particlebeam to the sample and obtains an image of the sample by at least one ofthe secondary electrons and backscattered electrons emitted from thesample and includes means which adjusts at least one of the probecurrent and the scanning speed of the primary charged particle beam sothat the scanning line density of the primary charged particle beambecomes 7.20×10⁻¹⁹ (C/nm) or less or a recommended value of 3.52×10⁻¹⁹(C/nm) or less, and the sample is scanned and observed.

Moreover, a scanning electron microscope in another mode according tothe present invention is a scanning electron microscope which radiatesthe primary charged particle beam to the sample and obtains an image ofthe sample by at least one of the secondary electrons and backscatteredelectrons emitted from the sample and provides means which adjusts atleast one of the probe current and the scanning speed of the primarycharged particle beam so that the scanning line density of the primarycharged particle beam becomes 7.20×10⁻¹⁹ (C/nm) or less or a recommendedvalue of 3.52×10⁻¹⁹ (C/nm) or less, an energy filter which disperses thesecondary electrons having energy at a specific value or more in thesecondary electrons emitted from the sample, means which extracts andrecords the charging relaxation time constant from the temporal changeof the secondary electron signal intensity measured by the energyfilter, and scanning-order determining means which determines thescanning order of the primary charged particle beam on the basis of thetime constant, and the sample is observed by the determined scanningmethod of the primary charged particle beam.

According to the present invention, charging caused by primary chargedparticle radiation is reduced during the observation, and the secondaryelectrons or backscattered electrons emitted from the sample canminimize the influence on the charging. Thus, improvement of the profileline intensity of the two-dimensional pattern including a non-metalmaterial and suppression of shading can be realized, and more stable andhighly accurate observation can be made.

Moreover, according to the present invention, as compared with theprior-art observation technologies, when an image is to be obtained,detection efficiency (secondary signal intensity/number of injectedelectrons of the primary charged particles in each pixel) of secondarysignals (secondary electrons and backscattered electrons) is high, andthus, an image having a similar signal/noise ratio can be obtained in ashorter time, which results in faster observation.

An image of a resist pattern illustrated in FIG. 2A photographed byusing the primary electron beam scanning line density and in-field linescanning-order control provided by the present invention is illustratedin FIG. 2D. Also, the profile extraction result thereof is illustratedin FIG. 2E. It was proved by experiments that profile intensity isimproved and accuracy of profile line extraction is improved thereby inan image of a two-dimensional pattern.

Further features of the present invention will be made clear below fromthe best mode for carrying out the present invention and the attacheddrawings.

Embodiment 1

[Configuration of Device]

An outline configuration of a scanning electron microscope in anembodiment of the present invention is illustrated in a block diagram inFIG. 5.

Between a cathode 1 and a first anode 2, a voltage is applied by ahigh-voltage control power supply 13 controlled by a calculating device22, and a predetermined emission current is drawn from the cathode 1.Since an acceleration voltage is applied between the cathode 1 and asecond anode 3 by the high-voltage control power supply 13 controlled bythe calculating device 22, a primary electron beam 4 emitted from thecathode 1 is accelerated and progresses to a lens system in the rearstage. The primary electron beam 4 is converged by a focusing lens 5controlled by a focusing-lens control power supply 14 in compliance withan instruction of an input device, deprived of an unnecessary region ofthe primary electron beam 4 by a diaphragm plate 7 and controls theprobe current Ip of the primary electron beam.

After that, the primary electron beam is converged on a sample 8 as amicro spot by an objective lens 6 controlled by an objective-lenscontrol power supply 15 and scans the sample by a polarizer 10 in atwo-dimensional manner A scanning signal of the polarizer 10 iscontrolled by a polarizer control power supply 16 in compliance withscanning conditions including the view-field size, scanning speed, andthe number of pixels specified by the input device 19. Also, the sample8 is fixed onto a sample stage 23 which is movable two-dimensionally.The movement of the sample stage 23 is controlled by a stage controlportion 17. A secondary electron 9 generated from the sample 8 byradiation of the primary electron beam 4 passes through an energy filter26 controlled by a control power supply 12 of the energy filter anddetected by a secondary electron detector 11, and a drawing device 20executes control of converting the detected secondary signal to avisible signal and aligning it on another plane as appropriate anddisplays an image corresponding to the surface shape of the sample on anSEM image display device 18 as an image.

The signal detected by the secondary electron detector 11 is amplifiedby a secondary signal amplifier 28 and then, accumulated in an imagememory in the drawing device 20. An address signal corresponding to amemory position in the image memory is generated in the calculatingdevice 22 or in a computer installed separately and converted to ananalog signal. Then, the address signal in the X-direction supplied tothe polarizer 10 is a digital signal repeating from 0 to 512 if theimage memory is raster scanning of 512×512 pixels, for example, whilethe address signal in the Y-direction is added with 1 when the addresssignal in the X-direction reaches 512 from 0 and is a digital signalrepeating from 0 to 512. This is converted to an analog signal.

Since the address in the image memory corresponds to the address of apolarization signal for scanning with the electron beam, atwo-dimensional image in an electron beam polarization region by thepolarizer 10 is recorded in the image memory. The signals in the imagememory can be sequentially read out in a time series by a reading-outaddress generation circuit synchronized by a reading-out clock. Thesignal read out in correspondence with the address is converted to ananalog signal and becomes a brightness modulation signal of the imagedisplay device 18.

The image memory is provided with a function of synthesizing andrecording image data for S/N improvement. For example, one completeimage is formed by overlapping and recording images obtained in 8sessions of two-dimensional scanning. That is, a final image is formedby synthesizing images formed by one session or more of the unit of X-Yscanning. The number of images (hereinafter referred to as the number ofcumulative frames) for forming one complete image can be arbitrarilyset, and a proper value is set, considering conditions such as secondaryelectron generation efficiency or the like.

The input device 19 realizes interface between an operator and thecalculating device 22, and the operator executes control of each of theabove-described units through this input device 19 and also specifies ameasurement point or gives an instruction of dimensional measurement.

Moreover, this device is provided with a line profile extractionfunction 24, which is means which extracts a line profile on the basisof the detected secondary electron or the like. The line profile isformed on the basis of a detected amount of the secondary electrons,brightness information of the image and the like in scanning with theprimary electron beam, and the obtained line profile is used fordimensional measurement or the like of a pattern formed on asemiconductor wafer, for example. In this Embodiment 1, the line profileis used in a function 25 for determining whether or not to performextraction of a pattern profile (function to determine whether or not toperform extraction of a pattern profile).

Moreover, in a storage device 21, pattern layouts to be inspected, edgeshape information, and observation recipes are stored.

[Charging Control Method]

An example of measurement of charging relaxation characteristics (timeconstant) of a sample is illustrated in FIG. 6.

When the primary electron beam 4 is radiated to the sample 8, asecondary signal 9 (including at least one of secondary electron andbackscattered electron) is generated. Since an electric field formed bythe sample holder 23, the objective lens 6, and an electrode 27 acts asan accelerating electric field to the secondary signal 9, it is pulledup into the passage of the objective lens 6 and rises while beingsubjected to the action of the magnetic field of the objective lens 6and further passes through the scanning polarizer 10 and enters theenergy filter 26. Depending on a set value of the energy filter 26, asecondary signal component having low motion energy cannot pass throughthe energy filter 26, while a component having motion energy higher thanthat passes through the energy filter 26. The primary electron beam 4 isradiated to the sample 8 with a certain dose amount, lets the sample 8charged, and an irradiation position potential Vs of the sample 8 ischanged. Here, the sample potential Vs is the sum of a charged potentialΔVs of the sample 4 generated by irradiation of the primary electronbeam 4 and a retarding potential Vr applied to the sample holder 23. Ifthe sample potential Vs is increased by charging, the amount of thesecondary signal which can pass through the energy filter 26 isdecreased, and the amount of the secondary signal detected by thedetector 11 is decreased, and brightness on the image is reduced. Theprimary electron beam 4 is radiated to the sample 8 in advance so as toform a charged region, the same region is irradiated again after acertain time interval, and the obtained brightness on the image isrecorded. FIG. 6 shows an example of a change curve of lightness of thecharged region on the screen when the time interval is changed. At thistime, the scanning method with the primary electron beam 4 may bearbitrarily set in compliance with the purpose (one-point irradiation,line scanning, two-dimensional scanning) Also, in order to raise S/N,the measurement is made at a plurality of spots on the sample, and theresult is averaged and outputted.

As illustrated in FIG. 7, which is a configuration diagram until theprimary electron beam scanning method is determined, first, the probecurrent and the scanning speed of the primary electron beam isdetermined so that the scanning line density of the primary electronbeam 4 becomes a predetermined value or less on the basis of theirradiation energy of the primary electron beam 4, the secondaryelectron and backscattered electron yield of the sample, and the upperlimits of the probe current and the scanning speed as illustrated in theformula (3). The scanning line density here is defined as a chargeamount injected per unit length on the sample.

Subsequently, the temporal change curve of the brightness is inputtedinto the time constant calculating device 29, and a charging relaxationtime constant of the sample 8 is extracted and stored in the storagedevice 21. The scanning method of the primary electron beam 4 isdetermined by the scanning method determining device 30 by using thestored charging relaxation time constant and the limitation conditionsof the primary electron scanning (including any of the number of imagepixels, view field, and the cumulated number). The scanning methodincludes the probe current and the scanning speed which determine thescanning line density of the primary electron beam 4 and the scanningorder in the view field corresponding to the scanning line density. Fordetermination of the scanning order, a similar method to the prior-arttechnologies may be used, for example. By using the determined scanningmethod or the scanning method selected by the operator from candidates,the primary electron beam 4 is used to scan the sample 8 so as to obtainan image and the sample is observed.

If the temporal change of the brightness is expressed asS(t)∝1−exp(−t/τ) (τ: charging relaxation time constant of sample), it isacquired by fitting with a curve illustrated in FIG. 6. The componentswhose brightness rapidly rises and enters a steady state are caused byextinction of an electron/hole pair in the sample or reunion of thesecondary electrons or backscattered electrons emitted once to theoutside of the sample with the hole on the sample or the like, and thistime constant is set to τ1. The component changing slowly is consideredto be caused by a leaking process through the surface of the sample orbulk, and this time constant is set to τ2. In a normal SEM observationcondition, it is τ1<τ2.

If SEM observation/photographing time is smaller than the time constantτ1, relaxation of the SEM observation/irradiation charging duringphotographing is small, and thus, influences on the pattern profile lineintensity and shading are large. If the SEM observation/photographingtime is between τ1 and τ2, it is possible to suppress the influences forthat portion by relaxation of the charging corresponding to τ1. If theSEM observation/photographing timing is longer than τ2, it becomespossible to suppress the influences of the both. In this embodiment, byproviding a control method of irradiation charging having the timeconstant τ equal to or less than the SEM observation/photographing timeand a scanning electron microscope using that, attention is paid to thetime of line scanning and the scanning method is determined so that thecharging with relatively short time constant is suppressed to theminimum, whereby an image with a high image quality capable oftwo-dimensional observation is obtained.

[Processing Sequence]

FIG. 8 is a flowchart for explaining a sequence for obtaining an imageaccording to this embodiment. At Step 100, it is determined whether theresult of measurement history of the previous charging relaxation timeconstant of the same type of sample or the same wafer is stored in thestorage device 21 or not, and need of charging relaxation time constantmeasurement is determined. If the charging relaxation time constant isto be measured, the electron beam is moved to charging relaxation timeconstant measurement spots at Step 101. The measurement spots of thecharging relaxation time constant is preferably a flat portion in thevicinity of the observation region or an observation portion or apattern portion equivalent to them.

Subsequently, at Step 102, a retarding potential, which is an energyfilter potential to be applied to the energy filter 26 is set andapplied to the energy filter 26. The energy filter potential is apotential to take in the secondary electron with high energy notaffected by local charging distribution on the sample. From Step 103 toStep 109, the primary electron beam 4 is radiated to the sample 8 so asto obtain the secondary signal 9 and the irradiation intermediate timerelationship. At Step 103, line scanning is performed on the sample witha constant dose amount and charging is generated. Waiting for a timeinterval of Δt (Step 104), the line scanning is performed from thecenter position on the line (Step 105) and moved to the subsequentmeasurement position (Step 107). Within a predetermined time t1 (Step106), the aforementioned Step 103 to Step 107 are repeated. Also, inorder to obtain constant S/N, the number of measurement sessions is setin advance (Step 108), and the measurement from the aforementioned Step103 to Step 107 is repeated.

At Step 110, data obtained till Step 109 is inputted into the timeconstant calculating device 29, and the charging relaxation timeconstant ‘ of the sample is calculated by the above-described method andstored in the storage device 21. At Step 111, on the basis of theextracted charging relaxation time constant’ and the limitationconditions for scanning with the primary electron beam 4, an optimalscanning method is determined by the method illustrated in FIG. 7. Ifthere are a plurality of candidate scanning methods, selection can bemade also by an operator. As an example, a list of scanning methodscorresponding to the charging relaxation time constant ‘ of the samplein each scanning line density is stored in the storage device 21 inadvance, and when the scanning line density and the charging relaxationtime constant’ are inputted, candidates of the scanning method aredisplayed and the method is determined by an operator.

At Step 112, an SEM image of the sample is obtained by using thescanning method outputted from the scanning method determining device,and observation is made. Also, the scanning method outputted here isstored together with the sample in the storage device so that an imagecan be obtained with the optimal scanning method without chargingrelaxation time constant measurement or optimization of the scanningmethod if the material, structure or pattern of the sample is consideredas equal in the subsequent observation.

If it is determined at Step 100 that measurement of the chargingrelaxation time constant τ is not required, the routine proceeds to Step111. At Step 111, if there is measurement history of the chargingrelaxation time constant with the equal sample in the past, the chargingrelaxation time constant is read out of the storage device 21. If thereis no measurement history, the constant is specified from the inputdevice 19 or a default value is used.

In this Embodiment 1, an example of the charging relaxation timeconstant is described, but a charging change characteristic timeconstant in electron beam irradiation may also be used.

Embodiment 2

This embodiment will be described using a device configurationillustrated in FIG. 9.

As compared with the device configuration of Embodiment 1 illustrated inFIG. 5, in the device configuration of Embodiment 2, a Kelvin probeforce microscope or a Kelvin probe is installed instead of the energyfilter for measuring the charging relaxation characteristics of a sampleand a control system thereof. The temporal change of the chargingrelaxation of the sample is measured through the probe control portion32 by using the Kelvin probe force microscope or the Kelvin method usingeither of them. The primary electron beam scanning method is determinedin the configuration diagram illustrated in FIG. 7 by using themeasurement data. A flowchart for obtaining an image according to thisembodiment is illustrated in FIG. 10. As compared with the flowchart inEmbodiment 1, the Kelvin probe force microscope or the Kelvin method isused instead of the method using the energy filter 26 for measurement oftemporal change of sample charging.

Embodiment 3

This embodiment will be described by using a flowchart illustrated inFIG. 11.

Starting at Step 201, a sample is loaded (Step 202). At Step 203,information relating to the sample is inputted or called from thedevice. At Step 203, a sample for observation is loaded, and materialinformation relating to electron beam irradiation charging is inputted.At Step 204, candidates of scanning methods recommended from the storagedevice 21 are determined on the basis of the sample information. At Step205, trial measurement positions are specified in order to furthernarrow the recommended scanning methods, and an image is obtained byusing each of the recommended scanning methods. At Step 206, patternprofile extraction processing is applied to the image obtained at theprevious step, and an extraction error rate is calculated. If there is ascanning method having the extraction error rate smaller than apredetermined value, formal observation is made by using the scanningmethod, and the processing is finished. If there are a plurality ofscanning methods that satisfy the conditions, the scanning method withthe minimum extraction error rate is used for the formal observation. Ifthere is no scanning method having the extraction error rate smallerthan the predetermined value, sample information for determining therecommended scanning methods is inputted again and retried, or theformal observation is made by using the scanning method with thesmallest extraction error rate in the trial measurement or the routineis finished without making measurement.

Alternatively, instead of the extraction error rate, the candidates forthe scanning method may be determined by setting a threshold value ofpattern edge contrast and by extracting the edge contrast from the imageobtained from each of the scanning methods and comparing it with the setthreshold value.

Alternatively, it may be so configured that S/N of an image is acquiredby the determined scanning method, the number of cumulative frames ofthe image in the formal observation is calculated and fed back to theimage obtaining method.

Embodiment 4

This embodiment will be described by using a flowchart illustrated inFIG. 12. At Step 301 to Step 303, a sample for observation is loaded,and material information relating to electron beam irradiation chargingis inputted. At Step 304, an optimal scanning method in pattern edgeintensity, uniformity and lightness uniformity is determined. Thedetermining method may be the method described in Embodiment 3. At Step305, a parameter which controls the primary electron beam is changedwith the recommended scanning method, and a series of images areobtained. At Step 306, analysis processing is applied to the imagesobtained at the previous step, a correction value of the parameter whichcontrols the state of the primary electron beam (focus state,astigmatism and the like, for example) is extracted, and feedback ismade to them. At Step 307, formal observation is made by using thecorrected primary electron beam.

Embodiment 5

Supplemental explanation of the contents of Embodiment 1 will be givenby using a GUI 401 illustrated in FIG. 13 and FIG. 8. When a userselects alignment 407 and positions of an arbitrary plurality of layout402, the alignment is processed by the calculating device 22. Thisprocessing is equivalent to the processing of S100 in FIG. 8. If it isYes at S100, the user selects calibration 408 in FIG. 13. At that time,information relating to the sample (process name of the sample when thesample is prepared, film thickness of the resist, material, pattern andthe like) is selected. After that, processing at S101 in FIG. 8 isconducted, and the processing is conducted through S111.

The recommended condition is displayed on a GUI 405 at S111 anddetermined by selection of the recommended condition by the user. Therecipe is prepared in accordance with the recommended condition,scanning is performed with the scanning line density corresponding tothe sample, and an SEM image is obtained (S112). By making measurementwith a suitable scanning line density in this way, particularly thehorizontal lines of the sample pattern can be measured. Also,operability for the user can be improved through the GUI.

This GUI is formed of the layout 402 of a sample 403, the “alignment”screen on which setting and calibration of position, inclination and thelike are made, a “calibration” screen for selection of a beam scanningcondition, a “beam calibration” screen on which the selected beamcondition is calibrated, a “recipe creation” screen on which setting ofpositions of test/measurement and a sequence is made, and a“measurement” screen on which test/measurement is conducted. On the“calibration” screen, means for inputting/selecting informationincluding a material and a structure of a wafer is provided so that auser can make an input. The device measures the charging characteristicsof the sample to be measured on a specific location on the sample andprovides a recommended scanning condition or calls up a recommendedscanning condition from a database. A trial measurement result by thosescanning conditions is displayed. On the basis of the result, thescanning condition to be used for the formal test/measurement isdetermined by the user or automatically.

Embodiment 6

This embodiment will be described by using a graph illustrated in FIG.14. FIG. 14 illustrates an SEM image of a space pattern in which a filmstructure is made of resist/BARC/Si substrate (hereinafter referred toas a parallel edge since the edge direction of the space is madeparallel with the line scanning direction when the SEM image isobtained) as an example. A line profile of the parallel edge extractedfrom the SEM image is also illustrated. A difference between the spaceportion and the maximum value of the white band is assumed to beparallel edge intensity S_(//). FIG. 14 illustrates dependency of theparallel edge intensity S_(//)/Ip of the resist pattern per unitincident electron by the electron beam scanning line density. If thescanning line density is 0.5 pieces/nm or less, it indicates thatS_(//)/Ip has a saturating tendency. When a pattern profile is extractedfrom an SEM image for two-dimensional measurement, an (allowable)erroneous extraction rate is different depending on an analysis tool ora measurement request. Before the test/measurement, the parallel edgeintensity illustrated in FIG. 14 or a parallel/perpendicular edgeintensity ratio is measured, and a scanning condition including theelectron beam scanning line density is selected in compliance with apredetermined threshold value. In our evaluation, by setting theelectron beam scanning line density to 2 pieces/nm or less (that is,3.2×10⁻¹⁹ (C/nm) or less), S_(//)/N_(p-p) becomes 1.1 or more, andextraction of a pattern profile is not obstructed.

REFERENCE SIGNS LIST

-   1 cathode-   2 first anode-   3 second anode-   4 primary electron beam-   5 focusing lens-   6 objective lens-   7 diaphragm plate-   8 sample-   9 secondary electron-   10 polarizer-   11 secondary electron detector-   12 energy filter control power supply-   13 high-voltage control power supply-   14 focusing-lens control power supply-   15 objective-lens control power supply-   16 polarizer control power supply-   17 stage control portion-   18 image display device-   19 input device-   20 drawing device-   21 device-   22 control calculating device-   12 sample stage-   24 line profile extraction function-   25 function of determining whether or not to extract pattern profile-   26 energy filter-   27 electrode-   28 secondary signal amplifier-   29 time constant calculating device-   30 scanning method determining device-   31 scanning line density setting device-   32 probe control portion-   33 probe for Kelvin probe force microscope or Kelvin probe-   401 GUI for test/measurement control-   402 layout of chip on wafer-   403 test/measurement target (wafer)-   404 measurement result of charging relaxation characteristics-   405 selection menu of recommended scanning condition-   406 selection portion of information of test/measurement wafer-   407 alignment selection portion-   408 calibration selection portion

1. A scanning electron microscope which forms an image of a scannedregion by scanning a two-dimensional region on a sample with an electronbeam, wherein: the scanning electron microscope suppresses a number ofinjected electrons injected on said sample by said electron beam perindividual single line scanned in a direction of said scanning bydetermining a probe current and a scan speed of scanning said individualsingle lines by said electron beam such that a scanning line density,which is a charge amount injected per unit of length on the sample byscanning of said electron beam, for each individual single line scannedby the electron beam on said sample is a predetermined value or lessbased on one or more electric characteristics of said sample derivedfrom said images, and said sample is scanned in individual single linesby the electron beam based on the determined probe current and scanspeed.
 2. The scanning electron microscope according to claim 1,comprising: means which inputs information of said sample, determineswhether an optimization procedure for a scanning strategy of saidelectron beam is necessary, provides candidate scanning method(s) tosaid sample upon determining the optimization procedure is necessary,and acquires images of said sample with the candidate scanningmethod(s), wherein at least one of the following information is derivedfrom said images taken by said scanning electron microscope: (1)brightness of the image(s), (2) contrast of edge(s) of features of saidsample, (3) uniformity of the brightness of said images, (4) uniformityof said contrast of said edge(s) in said image(s), and a signal-to-noiseratio of said image(s).
 3. The scanning electron microscope according toclaim 1, wherein candidates of recommended scanning methods areprovided; and wherein the electric characteristics of said sample and/orthe candidates of recommended scanning methods are stored, and whereinsaid candidates of recommended scanning methods for said electron beamare read out when the information concerning to said sample is input,and said electron beam scans over said sample using said scanningmethod(s) automatically and acquires images.
 4. The scanning electronmicroscope according to claim 1, wherein a sequence of plural spatiallyseparated line positions on said sample scanned by said electron beamare controlled.
 5. The scanning electron microscope according to claim1, wherein said electric characteristic further includes a chargingrelaxation time constant of said sample calculated based on a temporalchange of an intensity of secondary charged particles emitted from saidsample by radiating the electron beam to said sample.
 6. The scanningelectron microscope according to claim 1, wherein said predeterminedvalue is of said scanning line density is 7.2×10⁻′⁹ (C/nm).
 7. Thescanning electron microscope according to claim 1, wherein saidpredetermined value is of said scanning line density is 3.52×10⁻′⁹(C/nm).
 8. The scanning electron microscope according to claim 1,wherein the scan speed is increased from a normal scan speed.
 9. Thescanning electron microscope according to claim 1, wherein a focal pointor an astigmatism correction amount is calculated and the calculationresult is fed back to a charged particle optical system.
 10. A sampleobservation method in which an image of a scanned region is formed byscanning a two-dimensional region on a sample with the electron beam,comprising the steps of: suppressing a number of injected electronsinjected on the sample by said electron beam per individual single linescanned in a direction of said scanning by determining a probe currentand a scan speed of scanning said individual single lines by saidelectron beam such that a scanning line density, which is a chargeamount injected per unit of length on the sample by scanning of saidelectron beam, for each individual single line scanned by the electronbeam on the sample is a predetermined value or less based on one or moreelectric characteristics of said sample derived from said images; andscanning said sample in individual single lines by the electron beambased on the determined probe current and scan speed.
 11. The sampleobservation method according to claim 10, further comprising: stepswhich input information of said sample, determine whether anoptimization procedure for a scanning strategy of said electron beam isnecessary, provide candidate scanning method(s) to said sample upondetermining the optimization procedure is necessary, acquire images ofsaid sample with the candidate scanning method(s), wherein at least oneof the following information is derived from said images taken by saidscanning electron microscope: (1) brightness of the image(s), (2)contrast of edge(s) of features of said sample, (3) uniformity of thebrightness of said images, (4) uniformity of said contrast of saidedge(s) in said image(s), and a signal-to-noise ratio of said image(s).12. The sample observation method according to claim 10, furthercomprising the step of: providing candidates of recommended scanningmethods and acquiring images of said sample using said candidates ofrecommended scanning methods, wherein the electric characteristics ofsaid sample and/or the candidates of recommended scanning methods arestored, and wherein said candidates of recommended scanning methods forsaid electron beam are read out when the information concerning to saidsample is input, and said electron beam scans over said sample usingsaid scanning method(s) automatically and acquires images.
 13. Thesample observation method according to claim 10, wherein a sequence of aplural spatially separated line positions on said sample scanned by saidelectron beam are controlled.
 14. The sample observation methodaccording to claim 10, wherein said electric characteristic furtherincludes is a charging relaxation time constant of said samplecalculated based on a temporal change of an intensity of secondarycharged particles emitted from said sample by radiating the electronbeam to said sample.
 15. The sample observation method according toclaim 10, wherein said predetermined value is of said scanning linedensity is 7.2×10⁻¹⁹ (C/nm).
 16. The sample observation method accordingto claim 10, wherein said predetermined value is of said scanning linedensity is 3.52×10⁻¹⁹ (C/nm).
 17. The sample observation methodaccording to claim 10, wherein the scan speed is increased from a normalscan speed.
 18. The sample observation method according to claim 10,wherein a focal point or an astigmatism correction amount is calculatedand the calculated result is fed back to a charged particle opticalsystem.